testing: --run now allows filtering of subtests

Also fix typo in error message in sub_test.go from upstream,
and move a few members from B to common where they belonged.

Note that testdata/testing.go seems to be pushing the edge of what
the emulated cortex-m3 target can handle; using regexp in that test
causes it to fail on that target with an out of memory error.

TODO: once tinygo supports runtime.Goexit, consider just using upstream's testing directory...

src/testing/benchmark.go

@@ -77,9 +77,6 @@ type InternalBenchmark struct {
 // affecting benchmark results.
 type B struct {
 	common
-	hasSub       bool          // TODO: should be in common, and atomic
-	start        time.Time     // TODO: should be in common
-	duration     time.Duration // TODO: should be in common
 	context      *benchContext
 	N            int
 	benchFunc    func(b *B)